Fifth-last character is a winner (6)
I believe the answer is:
victor
'winner' is the definition.
(I know that winner can be written as victor)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for victor that I've seen before include "Oh dear!" , "winning player" , "''To the . . . . . . the spoils'' (6)" , "success" , "Conqueror" .)